In principle measuring this relationship between emissions and concentrations is straightforward; it is one of stock and flow.
As in this simple heuristic animation, this is much like a filling bath [stock] running a tap from which liquid pours into it [positive flow] and a plug-hole that drains that liquid away [negative flow]: -
In this very simplified ‘bath-analogy’, the: -
Emission-sources are like the tap [adding water to the bath]
Emission-sinks are like the plug-hole [subtracting water from the bath]
Atmosphere concentrations-level is like the bath level that results from the interplay between the flow-rates of the tap and the plug-hole.
Very simple measurement examples are like this: -
If the tap flows in at the same rate the plug-hole drains away, the bath-level will remain the constant, or
If the tap flows in at twice the rate the plug-hole drains away, the bath-level will rise at half the rate of the flow from the tap, or
If the plug-hole drains away at twice the rate of the flow from the tap, the bath-level will fall at half the rate of the drain from the plug-hole.
CBAT Domain One 'Contraction and Concentrations', is all about measuring the various permutations of 'stock-and-flow' between emissions and their atmospheric concentrations.